Beam focalization for ultrasonic transducers has been widely used to concentrate beams of acoustic longitudinal wave (LW) energy from an ultrasonic probe into a particular area of a part to be inspected. Beam focalization can efficiently compensate for energy losses at interfaces and within the part being tested, thereby increasing the signal to noise ratio of the ultrasonic detection and improving the accuracy of flaw sizing. One example of such focalization would be use of an ultrasonic probe to measure corrosion of the inner surface of a pipe. In this case a focused beam from the probe might be used to counteract defocusing at the outer surface of the pipe, in order that the resultant beam would be either parallel or focused on to the inner surface.
One method of focusing the ultrasonic beam is to employ a probe or probe array with a curved surface. The wedge is then manufactured with matching curvature, and the curvature of the probe-wedge interface may be customized in order to achieve the required focal properties. A problem with this approach is that it is difficult to achieve matching curvatures, and if the curvatures are mismatched, the probe surface is prone to be damaged if the probe is tightly screwed to the wedge. Another problem is that it is very expensive to manufacture probes with curved surface, and to customize the curvature for multiple applications. Probes with a flat active surface are much more economical.
Another method of focusing an ultrasonic beam is to use a composite wedge comprising two different materials having different LW velocity, wherein the focusing is achieved at the interface between the two different materials. Patent GB1285715 by Lack describes various combinations of ultrasonic generators and focusing wedges, in which the wedges include a lens made of a material whose acoustic velocity is different from that in the material of the rest of the wedge. However Lack is silent on the question of how such composite wedges may be assembled and how they may be manufactured in large quantities at reasonable cost.
Another problem not addressed by Lack is how to manage reflections of acoustic energy from the interface between the two different materials in the wedge. Reflections from the interface, followed by further reflection at the probe surface, can result in multiple acoustic reverberations which, particularly in cases where a single probe is used as both generator and receiver, can seriously interfere with reception of acoustic signals from flaws in the part being tested.